Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection, since this can increase fuel economy.
One approach to allow such operation in an engine is described in the U.S. Pat. No. 4,467,602. In such a system, different catalysts can experience different operating temperatures. As such, the approach in '602 uses a temperature sensor and provides rich engine operation to keep maintain emission control. Further, the system in '602 the two closest catalysts to the engine are at different distances from the engine.
However, the inventors have recognized a disadvantage with the approach described in '602. Specifically, during a cold engine start, it may take one of the closer catalysts longer to reach its operating temperature.
This can be overcome by operating the cylinder groups coupled to catalysts in different locations in a different way. For example, in one example, a method for controlling an engine with at least a first set of cylinders and a second set of cylinders is provided. The method comprises:
during an engine start, operating the second set of cylinders with an ignition timing more retarded than an ignition timing of said first set of cylinders, and
during engine operation, operating the first set of cylinders with injected fuel to carry out combustion and operating the second set of cylinders to induct air and without injected fuel.
In this way, it is possible to obtain the benefits of fuel cut operation, while having the ability to provide good emission control during engine starting.
In another aspect, the inventors herein have recognized another disadvantage with such the approach of '602. Specifically, such operation may maintain temperature of the catalysts, but overall emissions may still increase since the oxidation of rich gasses with lean gasses may be less efficient than operation about stoichiometry.
Another approach that could be used in such a system is to end fuel cut operation if catalyst temperature becomes too low and re-enable fuel cut cylinders to thereby provide stoichiometric operation before catalyst temperature drops.
However, the inventors herein have recognized a disadvantage with such an approach. Specifically, using temperature to prematurely end fuel cut operation can lead to less fuel economy improvement that otherwise could be available. Yet, if fuel cut-operation is continued beyond this point, then emission may increase when re-enabling fuel cut cylinders since one of the catalysts may be below its light-off temperature.
The above disadvantage can be overcome by an improved method to rapidly heat a catalyst that is cooled during fuel cut operation. For example, a method for controlling an engine with at least a first set of cylinders and a second set of cylinders, where a first catalyst is coupled to the first set and a second catalyst is coupled to both said first and second set downstream of said first catalyst, can be used. The method comprises:
operating the first set of cylinders with injected fuel to carry out combustion and operating the second set of cylinders to induct air and without injected fuel; and
after said operation, commencing fuel injection in said second set of cylinders, and during said commencing, operating said second set of cylinders with an ignition timing that is more retarded than an ignition timing of said first set, with said first set carrying out combustion about stoichiometry.
In this way, it is possible to provide an exhaust gas mixture about stoichiometry to the second catalyst, and at the same time provide most of the heat to the second catalyst which can then rapidly reach its operating temperature. In this way emission can be reduced and fuel economy improved by having the ability to extend fuel cut operation even if temperature of a catalyst falls.